Mount St Helens, the volcano in the northwest USA that erupted spectacularly in 1980, is showing signs of blowing again. But despite modern technology, predicting eruptions is still a black art, say US experts.

If it does blow, geologists are confident that the eruption would be more modest than the one 24 years ago, which killed 57 people. That blast removed the mountaintop, created the biggest landslide in recorded history, and deposited a blanket of ash as far away as Montana.

Scientists were alerted to activity brewing at Mount St Helens by a series of very small earthquakes that began on 23 September. These tiny shudders are thought to be caused by the underground movement of pressurized steam or gases released from rising magma.

Mounting tension

Since the 1980 eruption, a dome of lava nearly 300 metres high has built up within the crater at the mountain's summit. This forms a plug that seals off the vent to the magma chamber below.

Now magma seems to be rising in the vent. It's possible that the hot gases it will release might blow debris and ash out of the dome. There could be landslides and flows of debris from the crater, or even flows of molten lava.

On 29 September, the US Geological Survey's Cascades Volcano Observatory (CVO) placed Mount St Helens on alert-level two of a three-level system. This signifies that "processes are underway that have significant likelihood of culminating in hazardous volcanic activity".

The alert does not warn that people are at imminent risk, and the region around the volcano, in the Cascades mountain range, is mostly uninhabited. All the same, hiking and mountaineering close to the mountain have been banned temporarily, and the Washington Department of Natural Resources has stopped public access to all of its land within 12 miles of the volcano.

Imperfect science

As well as remote sensing of seismic tremors, researchers are assessing the volcano's activity by looking for ground movement on the dome, in the crater, or on the mountain's flanks. A team of scientists from the USGS has positioned instruments in the crater that allow changes in ground level to be detected by GPS (Global Positioning System) measurements from satellites.

This so-called geodetic monitoring has transformed scientists' ability to anticipate volcanic eruptions, according to John Eichelberger, a vulcanologist at the University of Alaska in Fairbanks. "We still haven't fully digested everything we can get out of the GPS," he says.

Eruption prediction also draws on satellite measurements of heat changes as rising magma warms up a volcano's crater. But it's still an imperfect science that involves subjective judgements, and is somewhat akin to "reading tea leaves", Eichelberger says. Spotting volcanic activity is one thing, but deciding whether it is going to create an eruption is quite another.

The current behaviour of Mount St Helens has some researchers scratching their heads though. There have been no sightings of gas being released from the dome, for example, which would be expected if magma is rising. Indeed, despite the rumblings, it is possible an explosion will not happen at all. "The activity could stop right now," says Eichelberger.